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Abstract:

Patterned active layers formed by nano-imprint lithography for use in
devices such as photovoltaic cells and hybrid solar cells. One such
photovoltaic cell includes a first electrode and a first electrically
conductive layer electrically coupled to the first electrode. The first
conductive layer has a multiplicity of protrusions and recesses formed by
a nano-imprint lithography process. A second electrically conductive
layer substantially fills the recesses and covers the protrusions of the
first conductive layer, and a second electrode is electrically coupled to
the second conductive layer. A circuit electrically connects the first
electrode and the second electrode.

Claims:

1. A device comprising:a first electrode;a first electrically conductive
layer formed by nano-imprint lithography and electrically coupled to the
first electrode, the first conductive layer defining a multiplicity of
protrusions and recesses;a second electrically conductive layer
substantially filling the recesses and covering the protrusions of the
first conductive layer;a second electrode electrically coupled to the
second conductive layer; anda circuit electrically connecting the first
electrode and the second electrode.

2. The device of claim 1, wherein one of the electrodes reflects
ultraviolet light and one of the electrodes is substantially transparent
to ultraviolet light.

3. The device of claim 1, wherein a spacing between the protrusions in the
first conductive layer is less than about 20 nm.

4. The device of claim 1, wherein a length of the protrusions in the first
conductive layer is at least about 50 nm.

5. The device of claim 1, wherein a ratio of the length of the protrusions
to the spacing between the protrusions is at least about 5.

6. The device of claim 1, wherein the first conductive layer or the second
conductive layer comprises a conductive polymer.

7. The device of claim 1, wherein the first conductive layer is an
electron acceptor layer and the second conductive layer is an electron
donor layer, or the first conductive layer is an electron donor layer and
the second conductive layer is an electron acceptor layer.

8. The device of claim 1, wherein the second conductive layer is formed by
electrochemical deposition.

9. The device of claim 1, wherein the first conductive layer or the second
conductive layer comprises a conductive polymer composition.

11. A nano-imprint lithography method comprising:forming a first
electrically conductive layer with a nano-imprint lithography process,
the first conductive layer having a multiplicity of protrusions and
recesses;depositing a second electrically conductive layer on the first
conductive layer, wherein depositing comprises substantially filling the
recesses in the first conductive layer and covering the protrusions in
the first conductive layer with the second conductive layer;electrically
coupling a first electrode to the first conductive layer;electrically
coupling a second electrode to the second conductive layer;
andelectrically connecting the first electrode and the second electrode.

12. The method of claim 11, wherein forming the first conductive layer
comprises solidifying a conductive polymerizable material on the first
electrode.

13. The method of claim 11, wherein depositing the second conductive layer
comprises electrochemically depositing the second conductive layer in the
recesses and on the protrusions of the first conductive layer.

14. The method of claim 11, wherein depositing the second conductive layer
comprises substantially filling the recesses such that the filled
recesses are substantially without voids.

15. The method of claim 11, wherein forming the first conductive layer
comprises forming a spacing of less than about 20 nm between the
protrusions.

16. The method of claim 11, wherein forming the first conductive layer
comprises forming the protrusions with a length of at least about 50 nm.

17. The method of claim 11, wherein forming the first conductive layer
comprises forming the protrusions with a ratio of the length of the
protrusions to the spacing between the protrusions of at least about 5.

18. The method of claim 11, wherein one of the electrodes reflects
ultraviolet light and one of the electrodes is substantially transparent
to ultraviolet light.

19. The method of claim 11, wherein the first conductive layer is an
electron acceptor layer and the second conductive layer is an electron
donor layer, or the first conductive layer is an electron donor layer and
the second conductive layer is an electron acceptor layer.

20. The method of claim 19, wherein forming the first electrically
conductive layer with a nano-imprint lithography process includes
ultraviolet curing of an organic conductive polymer to form the electron
donor layer.

21. A nano-imprint lithography method comprising:forming patterned layer
on a substrate, the patterned layer comprising a multiplicity of
protrusions;electrodepositing a conductive polymer on the patterned
layer; anddissolving the patterned layer to yield a conductive layer with
a multiplicity of recesses, wherein the recesses are complementary to the
protrusions of the patterned layer.

[0003]Nano-fabrication includes the fabrication of very small structures
that have features on the order of 100 nanometers or smaller. One
application in which nano-fabrication has had a sizeable impact is in the
processing of integrated circuits. The semiconductor processing industry
continues to strive for larger production yields while increasing the
circuits per unit area formed on a substrate; therefore nano-fabrication
becomes increasingly important. Nano-fabrication provides greater process
control while allowing continued reduction of the minimum feature
dimensions of the structures formed. Other areas of development in which
nano-fabrication has been employed include biotechnology, optical
technology, mechanical systems, and the like.

[0004]An exemplary nano-fabrication technique in use today is commonly
referred to as imprint lithography. Exemplary imprint lithography
processes are described in detail in numerous publications, such as U.S.
Patent Application Publication No. 2004/0065976, U.S. Patent Application
Publication No. 2004/0065252, and U.S. Pat. No. 6,936,194, all of which
are hereby incorporated by reference herein.

[0005]An imprint lithography technique disclosed in each of the
aforementioned U.S. patent application publications and patent includes
formation of a relief pattern in a formable (polymerizable) layer and
transferring a pattern corresponding to the relief pattern into an
underlying substrate. The substrate may be coupled to a motion stage to
obtain a desired positioning to facilitate the patterning process. The
patterning process uses a template spaced apart from the substrate and
the formable liquid applied between the template and the substrate. The
formable liquid is solidified to form a rigid layer that has a pattern
conforming to a shape of the surface of the template that contacts the
formable liquid. After solidification, the template is separated from the
rigid layer such that the template and the substrate are spaced apart.
The substrate and the solidified layer are then subjected to additional
processes to transfer a relief image into the substrate that corresponds
to the pattern in the solidified layer.

SUMMARY

[0006]In one aspect, a device includes a first electrode and a first
electrically conductive layer formed by nano-imprint lithography and
electrically coupled to the first electrode. The first conductive layer
defines a multiplicity of protrusions and recesses. A second electrically
conductive layer substantially fills the recesses and covers the
protrusions of the first conductive layer. A second electrode is
electrically coupled to the second conductive layer, and a circuit
electrically connecting the first electrode and the second electrode.

[0007]In another aspect, a nano-imprint lithography method includes
forming a first electrically conductive layer having a multiplicity of
protrusions and recesses with a nano-imprint lithography process. A first
electrode is electrically coupled to the first conductive layer. A second
electrically conductive layer is deposited on the first conductive layer,
and a second electrode is electrically coupled to the second conductive
layer. The first electrode and the second electrode are electrically
connected. Depositing may include substantially filling the recesses in
the first conductive layer and covering the protrusions in the first
conductive layer with the second conductive layer

[0008]In some implementations, one of the electrodes reflects ultraviolet
light and one of the electrodes is substantially transparent to
ultraviolet light. A spacing between the protrusions in the first
conductive layer may be less than about 20 nm, less than about 10 nm, or
less than about 5 nm. In some cases, a spacing between the protrusions in
the first conductive layer is between about 5 nm and about 20 nm, or
between about 5 nm and about 10 nm. A length of the protrusions in the
first conductive layer is at least about 50 nm, at least about 100 nm, at
least about 200 nm, at least about 300 nm, at least about 400 nm, or at
least about 500 nm. A length of the protrusions in the first conductive
layer may be less than about 1000 nm. A ratio of the length of the
protrusions to the spacing between the protrusions is at least about 5,
at least about 10, at least about 20, at least about 30, at least about
40, at least about 50, or at least about 100. A ratio of the length of
the protrusions to the spacing between the protrusions may be between
about 5 and about 100.

[0009]In some implementations, the first conductive layer or the second
conductive layer includes a conductive polymer composition. The polymer
composition may be organic or an organic-inorganic hybrid. An organic
polymer may be conjugated or non-conjugated. The conductive polymer
composition may include a polymer selected from the group consisting of
polyacetylene, polypyrrole, polythiophene, polyaniline, polyfluorene,
[6,6]-phenyl C61-butyric acid methyl ester, and combinations and
derivatives thereof. In some cases, forming the first electrically
conductive layer with a nano-imprint lithography process includes
ultraviolet curing of an organic conductive polymer to form the electron
donor layer.

[0010]In some implementations, the first conductive layer is an electron
acceptor layer and the second conductive layer is an electron donor
layer. In some implementations, the first conductive layer is an electron
donor layer and the second conductive layer is an electron acceptor
layer. The second conductive layer may be formed by electrochemical
deposition. In some cases, the second conductive layer is formed by vapor
deposition, spin coating, dip coating, or the like.

[0011]In some implementations, forming the first conductive layer includes
solidifying a conductive polymerizable material on the first electrode.
Depositing the second conductive layer may include electrochemically
depositing the second conductive layer in the recesses and on the
protrusions of the first conductive layer. Depositing the second
conductive layer comprises substantially filling the recesses such that
the filled recesses are substantially without voids.

[0012]In one aspect, a patterned layer including a multiplicity of
protrusions is formed on a substrate. A conductive polymer is
electrodeposited on the patterned layer, and the patterned layer is
dissolved to yield a conductive layer with a multiplicity of recesses,
wherein the recesses are complementary to the protrusions of the
patterned layer.

[0013]In one aspect, a nano-patterned layer is formed on a substrate with
a nano-imprint lithography process. The nano-patterned layer includes
protrusions and/or recessions. A conducting polymer is electrodeposited
between the protrusions, and the nanoporous patterned layer is
substantially removed to yield a nanoporous conducting layer including
conducting polymer. In some implementations, the substrate comprises
silicon. The nanoporous metal layer may include a metal oxide, such as
zinc oxide, aluminum oxide, or a mixture thereof. The conductive polymer
may include a polymer selected from the group consisting of
polyacetylene, polypyrrole, polythiophene, polyaniline, polyfluorene,
[6,6]-phenyl C61-butyric acid methyl ester, and combinations and
derivatives thereof.

[0014]In one aspect, a nanoporous patterned layer is formed on a substrate
with a nano-imprint lithography process. The nanoporous patterned layer
includes protrusions. A portion of the patterned layer is coated with a
conductive metal. A conductive polymer is electrodeposited on the
nanoporous patterned layer, and the nanoporous patterned layer is
substantially removed to form a conducting polymer layer on the
conductive metal layer.

[0015]In some implementations, coating the portion of the patterned layer
with the conductive metal includes coating a portion of the protrusions
with the conductive metal. Coating the portion of the patterned layer
with the conductive metal may include coating a portion of the
protrusions and the recesses between the protrusions with the conductive
metal. The conductive polymer layer may include nanowires or nanotubes.

[0016]In an aspect, a nanoporous patterned layer is formed by a
nano-imprint lithography method on a substrate. The nanoporous patterned
layer includes protrusions. A conductive polymer is electrodeposited on
the nanoporous patterned layer. In some implementations, the conductive
polymer includes polythiophene or other polymers described herein. The
nanoporous patterned layer may include n-type silicon.

[0017]In an aspect, a patterned layer is formed of a conductive material
on a substrate by an imprint lithography method. The patterned layer
includes protrusions. A portion of the patterned layer between the
protrusions is coated with an insulating material, and a conductive
polymer is electrodeposited on the protrusions of the patterned layer.

[0018]In some implementations, the electrodeposition occurs in the
presence of an electrolyte, and the insulating material is substantially
insoluble in the electrolyte. The insulating material may be
substantially removed from between the protrusions after
electrodepositing the conducting polymer.

[0019]In one aspect, a device includes a first electrode, an electron
acceptor layer formed by nano-imprint lithography electrically coupled to
the first electrode, the electron acceptor layer comprising recesses, an
electron donor layer electrochemically deposited in the recesses of the
electron acceptor layer, a second electrode electrically coupled to the
electron donor layer, and a circuit electrically connecting the first
electrode and the second electrode.

[0020]In one aspect, forming a photovoltaic device includes forming a
patterned electron acceptor layer with a nano-imprint lithography
process. The patterned electron acceptor layer includes recesses. An
electron donor layer is formed on the patterned electron acceptor layer.
Forming the electron donor layer includes electrochemically depositing an
electron donor in the recesses of the electron acceptor layer. A first
electrode is electrically coupled to the electron acceptor layer. A
second electrode is electrically coupled to the electron donor layer. The
first electrode and the second electrode are electrically connected.

[0021]In some implementations, the first electrode is transparent. The
first electrode may include indium tin oxide. The electron acceptor layer
may include silicon. A spacing between recesses in the electron acceptor
layer may be less than about 20 nm, and a depth of the recesses in the
electron acceptor layer is at least about 50 nm. A ratio of the depth of
the recesses to a spacing between the recess is at least about 5, at
least about 10, or at least about 20. The electron donor layer may
include a conductive polymer. The conductive polymer may be selected from
the group consisting of polyacetylene, polypyrrole, polythiophene,
polyaniline, polyfluorene, and combinations and derivatives thereof. In
some cases, the electron donor layer is formed from a liquid
polymerizable composition including a solvent, an electrolyte, or both.
The second electrode may be reflective, and may include aluminum, zinc,
cadmium, or other low work function metal.

[0022]In some implementations, electrochemically depositing the electron
donor includes substantially filling the recesses in the electron
acceptor from the bottom up. Electrochemically depositing the electron
donor may include substantially filling the recesses such that the filled
recesses are substantially without voids. Forming the electron donor
layer may include immersing the electron acceptor layer in a conductive
polymerizable liquid.

[0023]In one aspect, a device includes a first electrode, an electron
donor layer including recesses formed by nano-imprint lithography and
electrically coupled to the first electrode, an electron acceptor layer
deposited in the recesses of the electron donor layer, a second electrode
electrically coupled to the electron acceptor layer, the second electrode
including a conducting polymer, and a circuit electrically connecting the
first electrode and the second electrode.

[0024]In another aspect, forming a photovoltaic cell includes forming a
patterned electron donor layer including recesses with a nano-imprint
lithography process, forming an electron acceptor layer on the patterned
electron donor layer, wherein forming the electron acceptor layer
includes depositing an electron acceptor in the recesses of the electron
donor layer, forming a first electrode electrically coupled to the
electron donor layer, the first electrode including a conducting polymer,
forming a second electrode electrically coupled to the electron acceptor
layer, and electrically connecting the first electrode and the second
electrode.

[0025]In some implementations, the first electrode is transparent. The
first electrode may include a conductive polymer. Forming the first
electrode may include spin coating a polymerizable liquid on the electron
donor layer. The second electrode is reflective. The second electrode may
include PEDOT:PSS. Depositing the electron acceptor may include
electrochemically depositing the electron acceptor in the recesses of the
electron donor layer. he electron acceptor layer may include [6,6]-phenyl
C61-butyric acid methyl ester. The electron donor layer may include
a conducting polymer selected from the group consisting of polyacetylene,
polypyrrole, polythiophene, polyaniline, polyfluorene, and combinations
and derivatives thereof. The electron donor layer may be formed from a
liquid polymerizable composition in the absence of a solvent. The
electron donor layer is a may be a photopolymerization product of a
polymerizable composition including a conducting polymer precursor and a
cationic photoinitiator. Photopolymerization may include UV irradiation
at ambient temperature.

[0026]A spacing between recesses in the electron donor layer may be less
than about 20 nm. A depth of the recesses in the electron donor layer may
be at least about 50 nm. A ratio of a depth of the recesses to a spacing
between the recesses is at least about 5, at least about 10, at least
about 20, or at least about 30.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 illustrates a simplified side view of a lithographic system
in accordance with an embodiment of the present invention.

[0028]FIG. 2 illustrates a simplified side view of the substrate shown in
FIG. 1 having a patterned layer positioned thereon.

[0029]FIGS. 3A-3C depict the formation of a nano-patterned active layer
for a device such as a photovoltaic cell.

[0030]FIGS. 4A-4B illustrate cross-sectional views of a photovoltaic cell
with an active layer formed by nano-imprint lithography.

[0031]FIG. 5 illustrates electrochemical deposition of a conductive
polymer in recesses formed by nano-imprint lithography.

[0032]FIGS. 6A-6D illustrate formation of a conductive nanoporous film by
a process including nano-imprint lithography and electrodeposition.

[0035]FIGS. 8A-8C illustrate electrodeposition of a conducting polymer in
a nanoporous structure formed by nano-imprint lithography.

[0036]FIGS. 9A-9C illustrate electrodeposition of a conductive polymer on
exposed conductive regions of a patterned surface.

DETAILED DESCRIPTION

[0037]Referring to FIG. 1, illustrated therein is a lithographic system 10
used to form a relief pattern on substrate 12. Substrate 12 may be
coupled to substrate chuck 14. As illustrated, substrate chuck 14 is a
vacuum chuck. Substrate chuck 14, however, may be any chuck including,
but not limited to, vacuum, pin-type, groove-type, electromagnetic,
and/or the like. Exemplary chucks are described in U.S. Pat. No.
6,873,087, which is hereby incorporated by reference herein.

[0038]Substrate 12 and substrate chuck 14 may be further supported by
stage 16. Stage 16 may provide motion about the x-, y-, and z-axes. Stage
16, substrate 12, and substrate chuck 14 may also be positioned on a base
(not shown).

[0039]Spaced-apart from substrate 12 is a template 18. Template 18
generally includes a mesa 20 extending therefrom towards substrate 12,
mesa 20 having a patterning surface 22 thereon. Further, mesa 20 may be
referred to as mold 20. Template 18 and/or mold 20 may be formed from
such materials including, but not limited to, fused-silica, quartz,
silicon, organic polymers, siloxane polymers, borosilicate glass,
fluorocarbon polymers, metal, hardened sapphire, and/or the like. As
illustrated, patterning surface 22 comprises features defined by a
plurality of spaced-apart recesses 24 and/or protrusions 26, though
embodiments of the present invention are not limited to such
configurations. Patterning surface 22 may define any original pattern
that forms the basis of a pattern to be formed on substrate 12.

[0040]Template 18 may be coupled to chuck 28. Chuck 28 may be configured
as, but not limited to, vacuum, pin-type, groove-type, electromagnetic,
and/or other similar chuck types. Exemplary chucks are further described
in U.S. Pat. No. 6,873,087, which is hereby incorporated by reference
herein. Further, chuck 28 may be coupled to imprint head 30 such that
chuck 28 and/or imprint head 30 may be configured to facilitate movement
of template 18.

[0041]System 10 may further comprise a fluid dispense system 32. Fluid
dispense system 32 may be used to deposit polymerizable material 34 on
substrate 12. Polymerizable material 34 may be positioned upon substrate
12 using techniques such as drop dispense, spin-coating, dip coating,
chemical vapor deposition (CVD), physical vapor deposition (PVD), thin
film deposition, thick film deposition, and/or the like. Polymerizable
material 34 may be disposed upon substrate 12 before and/or after a
desired volume is defined between mold 20 and substrate 12 depending on
design considerations. Polymerizable material 34 may comprise a monomer
as described in U.S. Pat. No. 7,157,036 and U.S. Patent Application
Publication No. 2005/0187339, all of which are hereby incorporated by
reference herein.

[0042]Referring to FIGS. 1 and 2, system 10 may further comprise an energy
source 38 coupled to direct energy 40 along path 42. Imprint head 30 and
stage 16 may be configured to position template 18 and substrate 12 in
superimposition with path 42. System 10 may be regulated by a processor
54 in communication with stage 16, imprint head 30, fluid dispense system
32, and/or source 38, and may operate on a computer readable program
stored in memory 56.

[0043]Either imprint head 30, stage 16, or both vary a distance between
mold 20 and substrate 12 to define a desired volume therebetween that is
filled by polymerizable material 34. For example, imprint head 30 may
apply a force to template 18 such that mold 20 contacts polymerizable
material 34. After the desired volume is filled with polymerizable
material 34, source 38 produces energy 40, e.g., broadband ultraviolet
radiation, causing polymerizable material 34 to solidify and/or
cross-link conforming to shape of a surface 44 of substrate 12 and
patterning surface 22, defining a patterned layer 46 on substrate 12.
Patterned layer 46 may comprise a residual layer 48 and a plurality of
features shown as protrusions 50 and recessions 52, with protrusions 50
having a thickness t1 and residual layer 48 having a thickness
t2.

[0044]The above-described system and process may be further implemented in
imprint lithography processes and systems referred to in U.S. Pat. No.
6,932,934, U.S. Patent Application Publication No. 2004/0124566, U.S.
Patent Application Publication No. 2004/0188381, and U.S. Patent
Application Publication No. 2004/0211754, each of which is hereby
incorporated by reference herein.

[0045]Nano-imprint lithography may be used to form an active layer of a
photovoltaic cell. In an embodiment, an active layer of a photovoltaic
cell may be formed by solidifying a polymerizable composition to form a
patterned active layer on a substrate as described above with respect to
FIGS. 1 and 2. The patterned active layer may be an electron donor layer
or an electron acceptor layer. Nano-imprint lithography may be used to
achieve a desired spacing between electron donor material and electron
acceptor material.

[0046]FIGS. 3A-3C depict the formation of a nano-patterned conductive
polymer for a device such as an photovoltaic cell. The photovoltaic cell
may be an organic photovoltaic cell or hybrid solar cell. As depicted in
FIG. 3A, transparent mold 20, which may have release layer 21, is
oriented with respect to substrate 12. One or more layers 13 may be
present on the substrate. Layer 13 may be, for example, an adhesion
layer, a hard mask layer, or the like. A polymerizable composition 34 may
be applied to the substrate 12 (or additional layer 13) using, for
example, dispenser 35 to form a multiplicity of drops on the substrate.
The polymerizable composition 34 may include one or more polymer
precursors curable with ultraviolet light.

[0048]FIG. 4A illustrates a cross-sectional view of a portion of a
photovoltaic cell 400. Photovoltaic cell 400 includes electron donor
layer 402 and electron acceptor layer 404 sandwiched between transparent
electrode 406 and reflective electrode 408. Electron donor layer 402 and
electron acceptor layer 404 may include an electrically conductive
polymer composition. The conductive polymer composition may be organic
(e.g., carbon-containing and substantially non-metal-containing) or an
organic-inorganic hybrid (e.g., carbon-containing and metal-containing).
The conductive polymer may be conjugated or non-conjugated.

[0049]Electrical circuit 410 is formed between transparent electrode 406
and reflective electrode 408. Reflective electrode 408 is able to reflect
electromagnetic radiation present in solar energy and may include, for
example, aluminum, zinc, cadmium, and other low work function metals.
Transparent electrode 406 is substantially transparent to electromagnetic
radiation present in solar energy. Transparent electrode 406 may function
as an electron collection electrode. In an example, transparent electrode
402 is formed of glass coated with indium tin oxide. In another example,
transparent electrode 402 may include a conductive polymer such as
poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS).

[0050]An electrode made of doped conductive polymer with high
conductivity, high transparency to electromagnetic radiation, and a high
work function may be used as an anode in organic photovoltaic cells
including organic and organic-inorganic hybrid cells. The conductive
polymer electrode is non-rigid, and can be used in place of a more rigid
electrode with a lower work function, such as glass coated with indium
tin oxide. Conductive polymers that may be used as electrodes in
photovoltaic cells described herein include, for example, PEDOT:PSS and
other doped conjugated polymers with similar properties. In an example,
CLEVIOS PH500 (available from H.C. Starck, Germany), is a PEDOT:PSS that
can achieve a sheet resistance of less than 500 ohm/square and a
transmission of 75% at a thickness of 200 nm with one or more selected
polar solvents with a high boiling point (e.g., ethylene glycol).

[0051]Advantages of using conductive (e.g., conjugated) polymers as
electrodes for solar cells may include a high work function, which allows
efficient hole extraction. Other advantages include processability, which
allows better control of surface planarity, and increased adhesion
between layers (e.g., between polymer layers with similar chemical
properties). Electrodes formed from conductive polymers are
advantageously flexible (i.e., not rigid), allowing implementation in a
variety of configurations, including tandem cell arrays, V-shaped cells,
and the like, which may be used to enhance power conversion efficiency.
Additionally, fabrication costs for electrodes formed from conductive
polymers are advantageously less, than for electrodes formed from more
rigid, difficult to process materials.

[0052]Use of a conductive polymer as the anode in solar cells with a
nano-patterned active layer allows fabrication of solar cells from the
anode and the cathode. That is, the active layer may be formed on the
cathode (the reflective electrode) and the anode formed on the active
layer, or the active layer may be formed on the anode (the transparent
electrode), and the cathode formed on the active layer. One or more
conductive polymers or a mixture thereof can be deposited on the first
active layer by spin coating, inkjet printing, and the like, to form a
conductive, transparent electrode.

[0053]Referring again to FIG. 4A, protrusions 412 of electron donor layer
402 are interleaved with protrusions 414 of electron acceptor layer 404,
with a width of protrusions 414 defining a spacing S between protrusions
412, and a width of protrusions 412 defining a spacing S' between
protrusions 414. In some embodiments, the protrusions 412 and 414 are
substantially equal in width, and spacings S and S' are substantially the
same.

[0054]Spacings S and S' may be selected to be on the order of the distance
electrons and holes are able to diffuse through either the electron donor
material or the electron acceptor material, such that electrons are
transferred efficiently from the electron donor to the electron acceptor
and the holes in the photovoltaic cell are able to diffuse from an
acceptor layer to a donor layer. For some electron donor and electron
acceptor materials, the distance electrons are able to diffuse through
the material is less than about 20 nm (e.g., between about 5 nm and about
20 nm, or between about 10 nm and about 20 nm).

[0055]A depth of the recesses between protrusions 412 and 414, or a length
L of protrusions 412 and a length L' of protrusions 414, are selected
such that solar energy is efficiently captured. L and L' may be, for
example, at least about 50 nm, at least about 100 nm, at least about 200
nm, at least about 300 nm, or at least about 400 nm. In some cases, L and
L' are substantially the same. In photovoltaic cell 400, with S
substantially equal to S' and L substantially equal to L', a ratio of L/S
may be at least about 5, at least about 10, at least about 20, or
greater. Feature depths needed to absorb a sufficient quantity of solar
energy in organic solar cells is described by Gunes et al. in "Conjugated
polymer-based organic solar cells," Chemical Reviews, 107(4) 2007, pp.
1324-1338, which is incorporated by reference herein.

[0056]As shown in FIG. 4A, residual layer 420 of electron acceptor layer
404 is in contact with reflective electrode 408 and residual layer 418 of
electron donor layer 402 is in contact with transparent electrode 406.
This may be achieved by forming a patterned electron acceptor layer 404
on a reflective electrode 408 or by forming a patterned electron donor
layer 402 on transparent electrode 406. In some cases, however, as shown
in FIG. 4B, electron donor layer 402 is in contact with reflective
electrode 408, and electron acceptor layer 404 is in contact with
transparent electrode 406. This may be achieved by forming a patterned
electron donor layer 402 on a reflective electrode 408 or by forming a
patterned electron acceptor layer 404 on transparent electrode 406. In
some cases, rather than forming a pattern including protrusions and
rejections in, for example, an electron acceptor layer or an electron
donor layer, a multiplicity of recesses may be formed (e.g., etched) in
the layer.

[0057]In an example, patterned electron acceptor layer 404 may be formed
by a nano-imprint lithography process on reflective electrode 408.
Electron acceptor layer 404 may be formed by depositing polymerizable
electron acceptor material on reflective electrode 408 and forming
protrusions and recesses as described with respect to FIGS. 3A-3C. In
some cases, electron acceptor layer 404 is formed by using a nano-imprint
lithography process to etch a desired pattern in an electron acceptor
material, such as n-type silicon. Electron donor material may be
deposited in recesses 416 of electron acceptor layer 404 (e.g., between
or around protrusions 414 of the electron acceptor layer) to form
"protrusions" 412. Electron donor material may also be deposited on top
of the protrusions 414 of the electron acceptor layer to form "residual
layer" 418. Transparent electrode 406 may be formed on top of layer 418.

[0058]Electrochemical polymerization (or electropolymerization) may be
used to deposit one or more donor materials in recesses 416 in electron
acceptor layer 404 (e.g., between protrusions 414 in the electron
acceptor layer) to form "protrusions" 412. The donor material may include
a conductive polymer. In this process, a polymerizable liquid may be
placed in the recesses in the electron acceptor layer 404. In some cases,
the recesses are substantially filled with the polymerizable liquid. The
polymerizable liquid may include monomers capable of forming conductive
polymers with a low bandgap, such as polyacetylene, polypyrrole,
polythiophene, polyaniline, polyfluorene, and any combination or
derivative thereof. In an example, the polymerizable liquid includes
3-hexylthiophene, and the conductive polymer includes
poly-3-hexylthiophene.

[0059]The polymerizable liquid used to form the electron donor layer may
include a solvent, an electrolyte, one or more additional additives, or a
combination thereof. Examples of solvents include chlorobenzene,
acetonitrile, dichlorobenzene, water, and the like. Examples of
electrolytes include sulfuric acid, hypochlorite salts, and the like. If
a solvent is used, it may be selected to be compatible (e.g., miscible)
with the chosen monomers. Some monomers, for example, thiophene, may be
used a small amount of solvent or with no solvent.

[0060]In some embodiments, electron acceptor recesses are filled with
polymerizable liquid by immersing the acceptor layer in a polymerizable
liquid. The electron acceptor base may be used as the working electrode.
During electrochemical oxidation, the conductive polymer is
galvanostatically deposited in the nano-sized openings in the electron
acceptor layer 404. Deposition of the conductive polymer to form electron
donor layer 402 is achieved beginning at the bottom of recesses 416 in
the electron acceptor layer 404, as shown by the progression in FIGS.
5A-D. Electrodeposition is described by Hillman et al. in
"Electrochemistry of electroactive materials," Electrochimica Acta,
53(11) 3742-3743 (2008), which is incorporated herein by reference.

[0061]Deposition of the donor material from the bottom of the well up
allows the recesses in the acceptor layer to be filled with donor
material at an L/S ratio of up to about 400 substantially without the
formation of voids in the donor material. With the small spacing S
between acceptor and donor (e.g., about 5-20 nm), and the substantial
absence of voids in the acceptor material and the donor material, the
resulting photovoltaic cell demonstrates high conversion efficiency.

[0062]Referring again to FIGS. 3A-C, forming a patterned active layer by
imprint lithography may include photopolymerizing a polymerizable
composition including conductive polymer precursors and a cationic
photoinitiator to form an electron donor layer. The cationic
photoinitiators may be soluble in the polymer precursors (e.g.,
monomers). Thus, photopolymerization may be performed in the absence of a
solvent. In some cases, photopolymerization may occur in the presence of
a solvent such as, for example, tetrahydrofuran. Examples of conductive
polymer precursor/cationic photoinitiator combinations include pyrrole
and iron-arene salts, thiophene and iodonium salts, and the like.

[0063]Nano-imprinting of conductive polymer (e.g., electron donor or
p-type) materials with cationic photoinitiators may be achieved by UV
curing at room temperature. For example, p-type materials for organic
light emitting devices (OLEDs) and organic photovoltaic (OPV) cells can
be fabricated by UV curing of polymerizable compositions including
conducting (e.g., conjugated) polymer precursors and a cationic
photoinitiator. This process allows the formation, through nano-imprint
lithography, of a nano-patterned layer including features (e.g.,
nano-pillars, recesses, and the like) with a spacing of about 5-20 nm, or
on the order of the diffusing distance of charge carriers or excitons in
the conductive polymer.

[0064]FIG. 4B depicts an active layer formed by nano-imprint lithography,
sandwiched between a reflective electrode 408 and a transparent electrode
406. The p-type material of the electron donor layer 402 may include
polythiophene or other conductive polymers with a low bandgap. The n-type
layer of the electron acceptor layer 404 may include [6,6]-phenyl
C61-butyric acid methyl ester (PCBM) or other n-type materials. The
reflective electrode 408 may include, for example, aluminum. The
transparent electrode 406 may include a conductive polymer such as, for
example, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)
(PEDOT:PSS).

[0065]Recesses 422 in the electron donor (p-type) material may be spaced
about 20 nm apart or less (e.g., about 5-20 nm apart, or about 10-20 nm
apart). The recesses in the p-type material may be substantially filled
with electron acceptor (n-type) material that has been, for example, spin
coated, electrochemically deposited, or vapor deposited on the electron
donor material. The n-type material may substantially fill the recesses
in the p-type material and form a layer over the electron donor material.
A reflective electrode may be formed to substantially cover the electron
acceptor material. A transparent electrode may be formed on the electron
donor layer. An electrical circuit may be formed between the two
electrodes. A depth of the n-type material may be, for example, less than
a micron, but at least about 50 nm. A ratio of the depth of the n-type
material to the spacing between the p-type recesses may be at least about
5.

[0066]In some embodiments, a patterned layer on a conductive substrate may
be used as working electrode to guide the growth of conductive polymers
(e.g., polypyrrole, polythiophene, etc.) through electropolymerization.
The polymer may grow on a protrusion or in a recess, depending on the
conductivity of the protrusions and recesses. That is, polymerization may
occur in the area with less electrical resistance, defining nanotubes,
nanopillars, and the like.

[0067]After electropolymerization, the patterned layer, or template, may
be removed by treating with a suitable solvent, leaving holes to be
filled with an electron acceptor, such as PCBM. For example, a patterned
metal oxide layer may be removed by treating with acid. In some cases,
however, the template may be retained as a portion of the device. For
example, if a porous silicon wafer is used as a template, it can serve as
electron acceptor, with electropolymerized conductive polymer such as
polythiophene as the electron donor. When polymers are grown on
protrusions, templates may also be left intact.

[0068]Fabrication of photovoltaic cells by imprint lithography using
electropolymerization to directly deposit nanostructured conductive
polymers reduces handling of the polymerizable material and allows use of
a range of conductive polymers, including conductive polymers soluble in
common solvents. This method may be implemented without spin coating, and
thus without requirements of spin coating processes, including a wettable
surface. Additionally, the resolution or spacing between electron donor
portions and electron acceptor portions is advantageously governed by the
dimensions of the template formed by nano-imprint lithography, such that
high L/S ratios can be achieved.

[0069]When nanoporous templates (e.g., patterned layers with nano-sized
protrusions and recesses) with different conductivities in recesses and
protrusions are used as working electrodes to electrochemically
polymerize conductive polymer to define the nanostructure of active
materials in organophotovoltaic (OPV) cells, the cells provide high power
conversion efficiency and are relatively inexpensive to produce. This
method may also be used to fabricate other devices, including other
microelectronic devices such as organic light-emitting diodes (OLEDs).
Examples of the use of patterned layers or templates formed by
nano-imprint lithography to form conductive polymer structures are shown
in FIGS. 6-9.

[0070]FIGS. 6A-6C illustrate formation of a conductive layer with recesses
with a nano-imprint lithography process. As illustrated in FIG. 6A, a
patterned layer 602 with protrusions 604 may be formed by a nano-imprint
lithography process on a substrate 600. Patterned layer 602 may include,
for example, aluminum oxide, zinc oxide, titanium oxide, silicon oxide,
or the like. Substrate 600 may be a silicon substrate. In FIG. 6B
conductive polymer 606 is electropolymerized to fill the recesses in the
patterned layer and cover the nano-structures 604. Conductive polymer 606
may include, for example, polypyrrole. FIG. 6C illustrates dissolution of
the patterned layer, and thus separation of the patterned polymerized
layer 606 from the substrate 600. Recesses 608, defined by protrusions
604, remain in the conductive polymer layer 606. In an example, an acid
(e.g., phosphoric acid), may be used to dissolve the patterned layer 602.
In some cases, solvents other than acid may be used to dissolve the
patterned layer 602. The resulting conductive thin film 606 is shown in
FIG. 6D. Conductive thin film 606, with recesses 608, may be referred to
as a nanoporous conductive film, or a nanoporous thin film.

[0071]FIGS. 7A and 7B illustrate formation of nanotubes and nanowires by
electropolymerization. Formation of nanotubes and nanowires is described
by Cho et al. in "Fast Electrochemistry of Conductive Polymer Nanotubes:
Synthesis, Mechanism, and Application," Acc. Chem. Res., 2008, 41(6), pp
699-707, which is hereby incorporated by reference.

[0072]As shown in FIG. 7A, metal 700 (e.g., gold), may be coated on
protrusions 702 of a patterned layer. A conductive polymer 704 may be
electropolymerized proximate the metal, for example, along the surface of
the protrusions 702. The conductive polymer 704 may be, for example,
PEDOT. When the patterned layer is removed (e.g., dissolved, as shown in
FIG. 6C), the conductive polymer 704 remains in the form of nanotubes
706.

[0073]As shown in FIG. 7B, recesses 708 between protrusions 702 of a
patterned layer may be filled with a metal 700 (e.g., gold). A conductive
polymer 704 may be electrodeposited on the metal 700, between protrusions
702 of the template. When the template is removed (e.g., dissolved, as
shown in FIG. 6C), the conductive polymer remains in the form of
nanowires 710.

[0074]FIGS. 8A-8D illustrate electrodeposition of a conductive polymer in
a patterned (e.g., nanoporous) structure formed by nano-imprint
lithography. FIG. 8A shows a patterned layer 602 with protrusions 604. A
top view of patterned layer 602, shown in FIG. 8B, shows the nanoporous
structure of the patterned layer. The patterned layer 602 may be formed
from, for example, an inorganic semiconductor, such as n-type silicon. A
conductive polymer, such as polythiophene, may be electrodeposited on the
protrusions 604, filling the recesses between the protrusions to form
conductive layer 606, as shown in FIG. 8B.

[0075]As illustrated in FIGS. 9A-C, a patterned conducting layer may be
formed through nano-imprint lithography. The layer may include gold, for
example. As shown in FIG. 9A, an Insulating material 902 may be deposited
between protrusions 904 in gold layer 900. The insulating layer may be,
for example, wax, or any other insulating material that is substantially
insoluble in the electrolyte used for electrodeposition. As shown FIG.
9B, conductive polymer 906 may be electrodeposited on the exposed
protrusions 904. In FIG. 9C, the insulating material 902 may be dissolved
to expose the conductive recesses 908 between protrusions 904. In some
cases, however, the insulating material may be allowed to remain.

[0076]Further modifications and alternative embodiments of various aspects
will be apparent to those skilled in the art in view of this description.
Accordingly, this description is to be construed as illustrative only. It
is to be understood that the forms shown and described herein are to be
taken as examples of embodiments. Elements and materials may be
substituted for those illustrated and described herein, parts and
processes may be reversed, and certain features may be utilized
independently, all as would be apparent to one skilled in the art after
having the benefit of this description. Changes may be made in the
elements described herein without departing from the spirit and scope as
described in the following claims.

Patent applications by Fen Wan, Austin, TX US

Patent applications by Frank Y. Xu, Round Rock, TX US

Patent applications by Shuqiang Yang, Austin, TX US

Patent applications by Sidlgata V. Sreenivasan, Austin, TX US

Patent applications by Board of Regents, The University of Texas System

Patent applications by MOLECULAR IMPRINTS, INC.

Patent applications in class At least portion of which is transparent to ultraviolet, visible or infrared light

Patent applications in all subclasses At least portion of which is transparent to ultraviolet, visible or infrared light